Code and ciphers: Julius Caesar, the Enigma and the internet

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chapter 13
so
1�7�2�3
but, from the second line above, 3�17�2�7 and so
1�7�2�(17�2�7)�5�7�2�17
but, from the first line above, 7�3016�177�17 and so
or
1�5�(3016�177�17)�2�17�5�3016�887�17
887�17��1 (mod 3016).
We need a value of d that, when multiplied by 17, leaves remainder �1
when divided by 3016 and this is therefore �887, not �887. Now �887�
2129 (mod 3016) so, finally, the decipherment key, d, is 2129.
(Check: 2129�17�36 193�12�3016�1.)
The encipherment and decipherment processes in the RSA
system
To encipher a message using the RSA method the would-be user must
know the values of the modulus, n (�pq), and the encipherment key, e;
these are publicly available. Only the ‘owner’ of the system which is being
used knows the values of p and q and the decipherment key, d.
To encipher a message which is to be sent to the ‘owner’, and which he
alone can decipher, the user must:
(1) convert the letters of his message into numbers such as those given in
Table 1.1, so that, for example A�00, B�01, ..., Z�25;
(2) if the modulus n contains no more than D digits break the number
version of the message into blocks of no more than D digits; denote
these blocks by B 1 , B 2 , ...;
(3) encipher the blocks in order and independently by computing
(4) (B I ) e (mod n), I�1, 2, ..., giving cipher blocks C 1 , C 2 , ...
The cipher message is C 1 C 2 ... (a string of numbers, not letters).
To decipher a message the procedure is exactly the same as for encipherment
except that the blocks to be handled now are the cipher blocks C I and